Optical glass

ABSTRACT

The present invention provides optical glasses containing Bi 2 O 3  in which the optical glasses have at least one of the properties of being substantially free from opacification and being substantially devitrified within the glass body during reheating steps in production processes, superior chemical durability, and free from black coloring.  
     The optical glass has a refractive index (n d ) of no less than 1.75 and an Abbe number (ν d ) of no less than 10 as optical constants.  
     The optical glass contains Bi 2 O 3  in a content from no less than 10% by weight to less than 90% by weight, and has at least one of the properties of being substantially free from opacification and being substantially devitrified within the glass body under the conditions of a reheating test (a).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical glasses containing bismuthoxide, more particularly to optical glasses having at least one of theproperties of being free from opacification and being devitrified in aglass body under such conditions as press-molding, including precisionpresses and reheat presses as well as reheat tests thereof.

2. Related Art

In recent years equipments or instruments, equipped with opticalsystems, have been highly integrated and sophisticated, which leading tomore and more demands for the optical systems in terms of high accuracy,lightweight and miniaturization, thus the optical systems have beenmainly designed using aspheric lenses formed of high index/highdispersion glasses in order to reduce the number of lenses.

It is expensive and non-efficient in particular to produce asphericlenses by way of grinding or polishing processes; therefore, theaspheric lenses are presently produced by lower cost mass-productionprocesses without the grinding or polishing processes such that gobs orglass blocks are cut and grinded to form a preform material, the preformmaterial is heated and softened then is pressure-molded by use of a moldhaving a highly precise surface.

In order to attain the object to mass-produce the aspheric lenses withlower cost, it is necessary to investigate various conditions so as tosatisfy items (i) to (iii) below:

(i) the glass is free from devitrification i.e. maintains transparencyunder reheating conditions, for example, of reheating-pressing processesfor softening gobs or glass blocks by heating thereof;

(ii) the glass has superior chemical durability such that particularcontrol is unnecessary in handling thereof after the polishing step; and

(iii) the temperature at mold-pressing step is as low as possible, sothat molds for the mold-pressing can be far from surface oxidation andthus be repeatedly usable (there exists a relation between uppertemperatures at mold-pressing and transition temperatures; the progressof the surface oxidation may be slower as these temperatures beinglower).

With respect to (i) described above, the glasses based on TiO₂ or Nb₂O₅containing SiO₂ or B₂O₃ as a former tend to exhibit relatively highertransition temperatures or higher glass yield points. Accordingly, theseglasses are inappropriate for mass production, since crystals are likelyto deposit at reheating steps at producing aspheric lenses, whichcausing problems such as lowering of process yield.

On the other hand, Patent Literatures 1 and 2 disclose glasses based onP₂O₅ utilized as precision-press materials. These materials may besoftened and press-molded at temperatures lower than those ofconventional SiO₂ glasses. However, these glasses still exhibit higherglass transition temperatures, so that the glasses react with surfacesof mold materials, consequently optical parts come to difficult toreproduce the surface accuracy at the transferred surfaces through theprecision-molding processes, and also the surfaces of mold materialstend to be injured. Furthermore, these glasses are likely to cause thedevitrification due to basic components of P₂O₅, TiO₂, Nb₂O₅ or WO₃through the reheating step, and also are relatively difficult to undergoprecision press-molding due to problems such as possible fusion withmolds or their clacks.

In addition, Patent Literature 3 discloses a glass containing Bi₂O₃ as abasic component; however, the refractive index and the dispersion areinsufficient and also the glass transition point is higher. Furthermore,there exist such problems as the glass tends to display considerableopacification or to color into black at the reheating step in producingprocesses of the aspheric lenses or at reheating tests corresponding toreheat presses.

-   Patent Literature 1: Japanese Unexamined Patent Publication No.    07-97234-   Patent Literature 2: Japanese Unexamined Patent Publication No.    2002-173336-   Patent Literature 3: Japanese Unexamined Patent Publication No.    09-20530

The present invention has been made in light of the objects describedabove; that is, the present invention provides optical glassescontaining bismuth oxide in which the optical glasses are having atleast one of the properties of being free from opacification and beingdevitrified within glass body at the reheating step in producingprocesses of the aspheric lenses or at the reheating tests correspondingto the reheat presses, and also optical glasses with superior chemicaldurability and free from coloring into black.

SUMMARY OF THE INVENTION

In order to solve the problems described above, we have investigatedvigorously and found that a desirable glass can be obtained with lowerproduction costs that exhibits a high index/high dispersion such as 1.75or more of refractive index (n_(d)) and from 15 to 35 of Abbe number(ν_(d)) as optical constants, and has a glass transition point (Tg) ofno more than 550 degrees C., with at least one of the characteristics ofbeing free from opacification and being devitrified within the glassbody during the reheating step in the production processes of theaspheric lenses or during the reheating test corresponding to reheatpresses, and also being free from black coloring, such that the presentinvention has been completed. More specifically, the present inventionprovided as described below.

According to a first aspect of the present invention, an optical glassof the present invention has a refractive index (n_(d)) of no less than1.75 and an Abbe number (ν_(d)) of no less than 10 in terms of theoptical constants, in which the Bi₂O₃ content is from no less than 10%by weight to no more than 90% by weight, and has at least one of thecharacteristics of being substantially free from opacification and/orbeing devitrified within glass body under the condition of reheatingtest (a) shown below:

whereby, a test piece of 15 mm by 15 mm by 30 mm is reheated such thatthe test piece is heated from room temperature to a temperature of 80degrees C. higher than its transition temperature (Tg) for a period of150 minutes, maintained for 30 minutes at the temperature of 80 degreesC. higher than the glass transition temperature (Tg) of the opticalglass, allowed to cool to an ambient temperature, and finally observedvisually after polishing the opposing two sides of the test piece tothickness of 10 mm.

The optical glass according to the present invention may have at leastone of the characteristics of being free from opacification and beingdevitrified within glass body under the condition of reheating test (a),thus an optical glass may be provided which has at least one of theproperties of hardly opacifying and being devitrified even during thereheating step in the production process thereof.

In a second aspect of the optical glass as described in the first aspectof the present invention, the transmissivity loss is no more than 5% atrespective wavelengths of visible region in the reheating test (b) underthe following conditions:

whereby, a two side-polished test piece having a thickness of 10 mm isheated from room temperature to a yield point by increasing thetemperature at a rate of 6.5 degrees C. per second under a non-oxidizingatmosphere, being maintained at the yield point for 300 seconds, andlowering the temperature to 220 degrees C. by decreasing the temperatureat a rate of 2.4 degrees C. per second, and thereafter measuring thetransmissivity of the test piece to determine the transmissivity ofbefore and after the test.

The optical glass according to the present invention exhibits thetransmissivity loss of no more than 5% at respective wavelengths of thevisible region in the reheating test (b), thus an optical glass may beprovided which hardly turns black in color even during reheating stepsin production process thereof. The reason the glass turns black in coloris that the component of Bi₂O₃ turns into metal bismuth by action ofnon-oxidative gas when the glass material undergoes precisionpress-molding to produce an optical glass and the like. The term“respective wavelengths of visible region” as used herein means thewavelengths of 360 nm to 800 nm. The non-oxidative gas is preferablynitrogen gas, for example. The term “transmissivity loss” refers to theloss of transmissivity that is caused in the tested test piece comparedto the pre-test test piece through the reheating test (b).

In a third aspect of the optical glass as described in any one ofaspects one through three of the present invention, the value,calculated by dividing the transmissivity of the test piece after thereheating test (a) by the transmissivity of the test piece before thereheating test using a radiation (D ray) of wavelength 587.56 nm, is noless than 0.95.

In a fourth aspect of the optical glass as described in any one ofaspects one through three of the present invention, the difference in awavelength λ₇₀ of the test piece before the reheating test (a) and awavelength λ₇₀ after the reheating test is no more than 20 nm, where the“λ₇₀” refers to the wavelength at which the transmissivity being 70%.

According to the third and fourth aspects of the present invention, anoptical glass may be easily provided that exhibits less transmissivityvariation even at the reheating step in the production process thereof,since the value calculated from the transmissivities after and beforethe reheating test (a) is no less than 0.95, or the difference of λ₇₀ isno more than 20 nm between after and before the reheating test (a).

In a fifth aspect of the optical glass as described in any one ofaspects one through four of the present invention, the crystal depositcondition of the test piece after the reheating test (a) displays aninternal quality of a first or second grade and A or B grade withrespect to the evaluation which is in accordance with a measuring methodfor inclusion JOGIS13-1994.

According to this aspect of the present invention, an optical glass maybe easily provided with less foreign substances even during thereheating step in the production process thereof, by virtue of theinternal quality of the first or second grade and A or B grade evenafter the reheating test (a) by the evaluation which is in accordancewith the method of determining foreign substances JOGIS13-1994.

Concerning the description “internal quality of the first or secondgrade and A or B grade”, the first grade indicates that the total crosssection of foreign substances JOGIS13-1994 is less than 0.03 mm², andthe second grade indicates that the total cross section is from no lessthan 0.03 mm² to no more than 0.1 mm² on the basis of 100 ml. The Agrade indicates that the total number of the foreign substances is lessthan 10, and the B grade indicates from no less than 10 to less than 100on the basis of 100 ml.

In a sixth aspect of the optical glass as described in any one ofaspects one through five of the present invention, the transitiontemperature (Tg) of the glass is no more than 550 degrees C.

In accordance with this aspect, the temperature at mold pressing may beset at a lower temperature since the transition temperature (Tg) of theglass is no more than 550 degrees C. Accordingly, the reactivity betweenthe glass and molds can be reduced, thus the transmissivity degradationmay be easily suppressed, and at least one of the properties ofopacification and vitrification of the glass may be easily prevented.

In a seventh aspect of the optical glass as described in any one ofaspects one through six of the present invention the content of SiO₂ islower than the content of B₂O₃, the total content of SiO₂+B₂O₃ is fromno less than 1% by weight to no more than 60% by weight; and the totalcontent of TiO₂+Nb₂O₅+WO₃+RO+Rn₂O is no more than 60% by weight, inwhich R represents one or more elements selected from the groupconsisting of Zn, Ba, Sr, Ca and Mg; Rn represents one or more elementsselected from the group consisting of Li, Na, K and Cs.

In a eighth aspect of the optical glass as described in any one ofaspects one through six of the present invention, the content of SiO₂ islower than the content of B₂O₃, the total content of SiO₂+B₂O₃ is fromno less than 1% to no more than 60% by weight; and the total content ofTiO₂+Nb₂O₅+WO₃+RO+Rn₂O is from no less than 0.1% by weight to no morethan 55% by weight, in which R represents one or more elements selectedfrom the group consisting of Zn, Ba, Sr, Ca and Mg; Rn represents one ormore elements selected from the group consisting of Li, Na, K and Cs.

In a ninth aspect of the optical glass as described in any one ofaspects one through eight of the present invention, the content of SiO₂is lower than the content of B₂O₃, the total content of SiO₂+B₂O₃ isfrom no less than 1% by weight to no more than 60% by weight; and thetotal content of RO+Rn₂O is from no less than 0.1% by weight to no morethan 60% by weight, in which R represents one or more elements selectedfrom the group including Zn, Ba, Sr, Ca and Mg; Rn represents one ormore elements selected from the group including Li, Na, K and Cs.

According to the aspects seven to nine of the present invention,SiO₂+B₂O₃ and RO component+Rn₂O component may stabilize the glass andalso suppress transmissivity degradation in the reheating test (b).Accordingly, the optical glasses produced in these compositions mayeasily avoid transmissivity degradation through the reheating test.

In a tenth aspect of the optical glass as described in any one ofaspects one through nine of the present invention, the content of SiO₂is lower than the content of B₂O₃, the total content of SiO₂+B₂O₃ isfrom no less than 1% by weight to no more than 60% by weight, and thetotal content of Ln₂O₃+RO+Rn₂O is from no less than 0.5% by weight to nomore than 50% by weight, in which R represents one or more elementsselected from the group consisting of Zn, Ba, Sr, Ca and Mg; Rnrepresents one or more elements selected from the group consisting ofLi, Na, K and Cs; Ln represents one or more elements selected from thegroup consisting of La, Gd, Y, Ce, Eu, Dy, Yb and Lu.

According to this aspect of the present invention, the total content ofLn₂O₃+RO+Rn₂O within the range described above may lead to easystabilization of the glass.

In a eleventh aspect of the optical glass as described in any one ofaspects one through ten of the present invention, the content of MgO isless than 4% by weight, and the total content of TiO₂+Nb₂O₅+WO₃+Ln₂O₃ isno more than 10% by weight, in which Ln represents one or more elementsselected from the group consisting of La, Gd, Y, Ce, Eu, Dy, Yb and Lu.

According to this aspect of the present invention, the total content ofTiO₂+Nb₂O₅+WO₃+Ln₂O₃ within the range described above may suppress thetendency to increase devitrification which is induced by the componentMgO during the reheating test.

In a twelfth aspect of the optical glass as described in the eleventhaspect of the present invention, the content of Rn₂O is 0% by weight to1.5% by weight, in which Rn represents one or more elements selectedfrom the group consisting of Li, Na, K and Cs.

The alkaline metal components may be remarkably effective in stabilizingglasses and to lowering temperatures corresponding to thermalproperties; therefore, the content of Rn₂O within the range describedabove may bring about easy control of the glass water-resistance.Furthermore, the deterioration of moldability that is induced bypossible alkaline elution at precision press-molding steps may be easilyavoided.

In a thirteenth aspect of the optical glass as described in any one ofaspects one through twelve of the present invention , the total contentof Bi₂O₃+SiO₂+Al₂O₃+ZrO₂ is no less than 75% by weight.

According to this aspect of the present invention, the total content ofBi₂O₃+SiO₂+Al₂O₃+ZrO₂ within the range described above may allow controlof improvement of chemical durability, along with satisfying therefractive index of the glass. In addition, it may easily suppresscoloring during mold-pressing. When a glass is to be produced withsuperior water resistance in particular, it is effective to raise thecontent of Bi₂O₃ and SiO₂; and when the acid resistance is to beimproved, it is effective to decrease the content of Bi₂O₃ and toincrease the content of Al₂O₃ and ZrO₂.

In a fourteenth aspect of the optical glass as described in any one ofaspects one through thirteen of the present invention, the weight lossof the glass is no more than 0.2% by weight in terms of chemicaldurability test based on powder method in accordance with JOGIS06-1996.

According to this aspect of the present invention, the glass may beprevented from turning black in color when the glass undergoes precisionpress-molding to produce optical glasses and the like since the weightloss of the glass is no more than 0.2% by weight in terms of chemicaldurability test based on a powder method in accordance withJOGIS06-1996. Furthermore, the resulting glasses may be easily preventedfrom degradation at rinsing steps or during storage of the opticalglasses, and also the transmissivity degradation may be easily preventedafter forming lenses.

The term “chemical durability” refers to a decay in durability againstglass corrosion induced by water, which may be determined by way of a“Method of Determining Chemical Durability of Optical Glass”JOGIS06-1996 as specified by Japanese Optical Glass IndustrialStandards. The method rates samples into 6 steps of class 1 to class 6from their weight loss on the basis of their weights before and afterthe test. The glass weight loss of 0.2% corresponds to a waterresistance equal to or superior than class 3. In the method, class 1represents less than 0.05% by weight of the weight loss on the basis ofthe weights before and after the test, class 2 represents from no lessthan 0.05% by weight to less than 0.10% by weight, class 3 representsfrom no less than 0.10% by weight to less than 0.25% by weight, class 4represents from no less than 0.25% by weight to less than 0.60% byweight, class 5 represents from less than 0.60% by weight to less than1.10% by weight, and class 6 represents no less than 1.10% by weight.

In a fifteenth aspect of the optical glass as described in any one ofaspects one through fourteen of the present invention, the value of(SiO₂+Al₂O₃+ZrO₂)/RO is no less than 0.5.

According to a sixteenth aspect of the present invention, an opticalelement formed by precision press-molding the optical glass according toany one of aspects one to fifteen.

According to the present invention, an optical element may be easilyprovided by way of precision press-molding since the optical glasshardly causes devitrification and also hardly colors into black evenafter reheating thereof.

In a seventeenth aspect of the optical glass as described in any one ofaspects one through fifteen of the present invention, the preform isutilized for precision press-molding.

According to the sixteenth aspect of the present invention, an opticalelement formed by precision press-molding the preform for precisionpress-molding as described in the seventeenth aspect.

According to the seventeenth and eighteenth aspects of the presentinvention, the preform may be effectively utilized for precisionpress-molding since the preform is free from devitrification or coloringeven after reheating thereof, thus the optical element may be easilyproduced by precision press-molding the preform for precisionpress-molding.

The optical glass according to the present invention may be provided asexcellent optical glasses in terms of preform productivity, propertiesof preform itself, and press-molding property by virtue of employing theconstitutional elements described above, furthermore, optical glassesmay be easily provided with generally excellent properties includingremarkably appropriate press-molding property.

DETAILED DESCRIPTION OF THE INVENTION

The optical glass according the present invention will be explained withrespect to specific embodiments below.

Glass Component

The composition range of respective components that constitute theinventive optical glasses will be explained in the following. Therespective components are expressed in terms of % by weight. The glasscompositions expressed in this specification are entirely on the basisof oxide. The term “on the basis of oxide” means that the contents ofthe respective components are expressed assuming that the raw materialsof the inventive glasses such as oxides and nitrates are entirely turnedinto oxides through decomposition and the like at the melting step andthe total content of resulting oxides is 100% by weight.

Essential and Optional Components

Bi₂O₃ is an essential component in order to attain the object of thepresent invention since it is effective to stabilize glasses, to achievehigh index/high dispersion and to lower glass transition temperatures(Tg). Excessively higher amount of Bi₂O₃, however, tends to degradeglass stability, and excessively lower amount of Bi₂O₃ makes difficultto attain the inventive object. Accordingly, the content of Bi₂O₃ ispreferably no less than 10%, more preferably no less than 20% and mostpreferably no less than 30%, preferably less than 90%, more preferablyno more than 85% and most preferably no more than 80%.

B₂O₃ or SiO₂ is an essential component as a glass-forming oxide, andsignificantly effective for the devitrification and to raise theviscosity at the liquid-phase temperature. The lower-limit content ofone of these components or sum of these components is preferably 1%,more preferably 3%, and still more preferably 7%. On the other hand, theupper-limit content thereof is preferably 60%, more preferably 50% andstill more preferably 40% in order to achieve desirable refractiveindexes.

These two components may exhibit an effect to improve devitrificationresistance even when one of these is introduced alone in the glasses,thereby the object of the present invention may be attained. When theratio SiO₂/B₂O₃ is controlled to less than 1.0 (the content of SiO₂being less than that of B₂O₃), the devitrification resistance may befurther improved within glass body.

When a glass yield point (At) according to the inventive object is to beattained effectively, the upper-limit content of B₂O₃ is preferablydefined as 30%, more preferably 25% and most preferably 20%. Inaddition, the upper-limit content of SiO₂ is preferably defined as 20%,more preferably 15% and most preferably 10%.

Al₂O₃ is an effective component to improve chemical durability; however,excessively higher content thereof tends to deteriorate glasssolubility, to increase devitrification, and to raise the glass yieldpoint. Accordingly, the upper-limit content is preferably defined as20%, more preferably 15% and most preferably 10%.

TiO₂ is an effective optional component in the glasses to raise therefractive index, to contribute to the high dispersion, and to lower theliquid-phase temperature; however, excessively higher content thereoftends to disadvantageously prompt the devitrification. Accordingly, thecontent is preferably defined as no more than 20%, more preferably nomore than 10% and most preferably no more than 5%.

Nb₂O₅ is an effective optional component in the glasses to raise therefractive index, to contribute to the high dispersion and to improvedevitrification; however, excessively higher content thereof tends todeteriorate the glass solubility. Accordingly, the content is preferablydefined as no more than 20%, more preferably no more than 15% and mostpreferably no more than 8%.

WO₃ is an effective optional component in the glasses to raise therefractive index, to contribute to the high dispersion and to lower theyield point; however, excessively higher content thereof tends toincrease phase-separation in glasses. Accordingly, the content ispreferably defined as no more than 15%, more preferably no more than10%, and most preferably no more than 5%.

Ta₂O₅ is an effective optional component in the glasses to raise therefractive index and to improve to the chemical durability; however,excessively higher content thereof tends to increase phase-separation inthe glasses. Accordingly, the upper-limit content is preferably definedas 15%, more preferably 10% and most preferably 5%. Still morepreferably, there exists no Ta₂O₅.

ZrO₂ is an optional component effective to improve to the chemicaldurability; however, excessively higher content thereof tends to promoteinclination to the devitrification of the glasses. Preferably, theupper-limit content is defined as 10%, more preferably 5% and mostpreferably 2%. Still more preferably, there exists no ZrO₂.

As described above, Al₂O₃ and ZrO₂ are effective components forimproving the chemical durability; Bi₂O₃ provides an effect to enhancethe water resistance. SiO₂ is an essential component as a glass-formingoxide which is significantly effective for the devitrification and forraising the viscosity at the liquid-phase temperature. Accordingly, itis preferred that these components are controlled in a certain range soas to satisfy the devitrification and the chemical durability of theglasses. In addition, the present inventors have found that there is anintimate relation between the transmissivity degradation and the waterresistance in precision press-molding products, that is, the enhancementof water resistance or establishment of firm glass construction maysignificantly contribute to mitigate the transmissivity degradation atprecision pressing steps. As such, when the total content of thesecomponents is excessively lower, the glass is likely to cause thecoloring at heating under non-oxidative atmosphere and also to degradethe devitrification resistance. Accordingly, the lower limit of thetotal content of Bi₂O₃, SiO₂, Al₂O₃ and ZrO₂ is preferably 65%, morepreferably 70% and most preferably 75%.

In addition, the RO component described later is an optional componentthat may provide mainly an effect to stabilize the glasses; when anoptical constant is to be controlled, it may be considered as areference for the entire composition. That is, when the refractive indexto be raised by use of a component such as Bi₂O₃ or to be lowered by useof other components, a portion of the RO component is often substitutedby a component. As such, the chemical durability and the glass stabilitymay be properly satisfied by way of setting the total content of SiO₂,Al₂O₃ and ZrO₂ while considering the RO as a reference. Accordingly, theratio of the total content of SiO₂, Al₂O₃ and ZrO₂ to the RO componentis preferably no less than 0.5, more preferably no less than 0.6, andmost preferably no less than 0.7.

The RO component, in which R represents one or more elements selectedfrom the group consisting of Zn, Ba, Ca, Mg and Sr, may increase themelting property and the devitrification resistance and to enhance thechemical durability, thus the glasses preferably contain any of thesecomponents. Preferably, the glasses contain the RO, in which Rrepresents one or more elements selected from the group consisting ofZn, Ba, Ca, Mg and Sr in a total content of no less than 0.1%, morepreferably no less than 5% still more preferably no less than 10%.

ZnO is an effective component to improve to the chemical durability;however, excessively higher content thereof tends to allow thedevitrification of the glasses. Accordingly, it is preferred that theupper-limit content is defined as 20%, more preferably 15% and mostpreferably 10%.

CaO is an effective component to improve to the melting property of theglasses; however, excessively higher content thereof tends to allow thedevitrification. Accordingly, it is preferred that the upper-limitcontent is defined as 20%, more preferably 15% and most preferably 10%.

BaO is an effective component to improve the devitrification and thecoloring of the glasses; however, excessively higher content thereof maydisturb the refractive index intended by the present development.Accordingly, it is preferred that the upper-limit content is defined as50%, more preferably 40% and most preferably 35%. The lower-limitcontent is preferably defined as 0.1%, more preferably 1% and mostpreferably 3%.

MgO is an effective component to attain the high dispersion of theglasses; however, excessively higher content thereof may promote theoccurrence of the devitrification at the reheating test. Accordingly, itis preferred that the upper-limit content is defined as less than 10%,more preferably less than 7% and most preferably less than 4%.

SrO is an effective component to improve the devitrification property ofthe glasses; however, excessively higher content thereof may makedifficult to attain the intended optical constant. Accordingly, it ispreferred that the upper-limit content is defined as 50%, morepreferably 40% and most preferably 35%.

The Rn₂O component, in which Rn represents one or more elements selectedfrom the group consisting of K, Na, Li and Cs is an effective optionalcomponent in the glasses to lower the melting property and the glassyield point; however, excessively higher content thereof may promote thetransmissivity degradation at heating under non-oxidative atmosphere.Accordingly, it is preferred that the upper-limit content is defined as10%, more preferably 5% and most preferably 1.5%.

Li₂O is an effective component in the glasses to improve the meltingproperty and to prevent the occurrence of devitrification at thereheating test; however, excessively higher content thereof may makedifficult to take the refractive index intended in the presentinvention. Accordingly, it is preferred that the upper-limit content isdefined as 15%, more preferably 10% and most preferably 5%.

Na₂O is an effective component in the glasses to improve thedevitrification property and to prevent the occurrence ofdevitrification at the reheating test; however, excessively highercontent thereof may lower the refractive index. Accordingly, it ispreferred that the upper-limit content is defined as 15%, morepreferably 10% and most preferably 5%.

K₂0 is an effective component in the glasses to improve thedevitrification property; however, excessively higher content thereofmay make difficult to take the refractive index intended in the presentinvention. Accordingly, it is preferred that the upper-limit content isdefined as 20%, more preferably 15% and most preferably 10%.

Furthermore, in order to improve the devitrification property intendedin the present invention, it is preferred that the lower-limit contentof RO+Rn₂O is defined as 0.1%, more preferably 5% and most preferably10%. The upper-limit is preferably defined as 60%, more preferably 55%and most preferably 50%.

TiO₂, Nb₂O₅ and WO₃ are significantly important components to controlthe optical constant as described above, and these contents arepreferably adjusted to certain levels while keeping a relation with ROand/or Rn₂O components. When the total content is excessively higher,the devitrification tends to develop significantly, thus the glassstability may be degraded remarkably. Accordingly, the upper-limit ofTiO₂, Nb₂O₅, WO₃, RO and Rn₂O is preferably 60% in terms of their totalcontent, preferably 55% and most preferably 50%. The lower-limit ispreferably no less than 0.1%, and 0% is allowable.

When the content of TiO₂, Nb₂O₅ and WO₃ is excessively high as for thetotal content with Ln₂O₃, the glass stability may be degradedremarkably. Accordingly, the upper-limit of TiO₂, Nb₂O₅, WO₃ and Ln₂O₃is preferably 60% in terms of their total content, preferably 40% andmost preferably 10%.

The components of Y₂O₃, La₂O₃, Gd₂O₃ and Yb₂O₃ are effective in theglasses to enhance the chemical durability, and these components may beadded optionally. When the content is excessively higher, the dispersiontents to be deteriorated and the devitrification resistance tends toincrease. Accordingly, the upper-limit of these components is preferablydefined as 10% in terms of their total content, more preferably 7% andmost preferably 0.1%. Still more preferably, there exists no thesecomponents.

In addition, it is preferred that the total content of Ln₂O₃, in whichLn represents one or more elements selected from the group consisting ofLa, Gd, Y, Ce, Eu, Dy, Yb and Lu is controlled within a rangeconsidering the relation with the content of RO and/or Rn₂O components.The upper limit of Ln₂O₃, RO and Rn₂O is preferably 50% in terms oftheir total content, more preferably 48% and most preferably 45%. Thelower-limit is preferably 0.5%, more preferably 1% and most preferably1.5%.

P₂O₅ is a component effective to improve the coloring in the glasses,and the component may be added optionally. Excessively higher contentthereof tends to promote phase-separation of the glasses. Accordingly,the upper-limit of the component is preferably defined as 10%, morepreferably 5%, and most preferably 1%. Still more preferably, thereexists no this component.

Sb₂O₃ may be optionally added for defoaming the melted glasses, andprovides the effect sufficiently in a content of no more than 3%.

GeO₂ is an effective component in the glasses to improve the coloringand to enhance the high index/high dispersion, and is added in somecases considering its relatively higher cost. Accordingly, theupper-limit of the component is preferably defined as 20%, morepreferably 10% and most preferably 5%. Still more preferably, thereexists no this component.

F may affect to enhance the melting property of the glasses, and mayoptionally be added since it drops the refractive index drastically.Accordingly, the upper-limit of the component is preferably defined as5%, more preferably 3% and most preferably 1%. Still more preferably,there exists no this component.

Components Non-Desirable to Include

The other components may be added as required provided that theproperties of the inventive glasses are not deteriorated. In thisregard, components of various transition metals such as V, Cr, Mn, Fe,Co, Ni, Cu, Ag and Mo, except for Ti, induce coloring of the glasseseven when included at a small amount individually or in combinationthereof, thereby causing radiation absorption at a certain visiblewavelength. Accordingly, it is desirable for optical glasses employed atvisible wavelengths to contain substantially no these components.

Th component may be included for the purpose of raising the refractiveindex and stabilizing the glasses, and Cd and Tl components may beincluded for the purpose of lowering the glass transition temperature(Tg). However, in these years the components of Pb, Th, Cd, Tl and Osare likely to be avoided from their usage in light of harmful chemicalsubstances, and environmental measures are required not only at theproduction steps of the glasses but also at processing steps anddisposal of the produced articles. Accordingly, it is preferred thatthese components are substantially excluded when the environmentaleffects are important.

The component of lead requires the environmental measures at producing,processing and disposing of the glasses, resulting in higher cost, thusthe lead is to be nothing within the inventive glasses.

As₂O₃ is a component to improve the defoaming property at melting theglasses; however, it requires the environmental measures at producing,processing and disposing of the glasses, thus it is undesirable toinclude As₂O₃ in the inventive glasses.

In accordance with the present invention, it is preferred that at leastone of the following components is included, as indicated:

Bi₂O₃: 10 to less than 90%,

SiO₂: more than 0% to less than 20%,

BaO: 0 to 50%,

B₂O₃: 0 to 30%,

Al₂O₃: 0 to 20%,

TiO₂: 0 to 20%,

Nb₂O₅: 0 to 20%,

WO₃: 0 to 15%,

Ta₂O₅: 0 to 15%,

ZrO₂: 0 to 10%,

ZnO: 0 to 20%,

MgO: 0 to less than 10%,

CaO: 0 to 20%,

SrO: 0 to 50%,

Li₂O: 0 to 15%,

Na₂O: 0 to 15%,

K₂O: 0 to 20%,

Y₂O₃: 0 to 10%,

La₂O₃: 0 to 10%,

Gd₂O₃: 0 to 10%,

Yb₂O₃: 0 to 10%,

P₂O₅: 0 to 10%,

Sb₂O₃: 0 to 3%,

GeO₂: 0 to 20%, and

F: 0 to 5%.

The optical glasses according to the present invention are of highindex/high dispersion, and may easily display a glass transitiontemperature (Tg) of no more than 550 degrees C. Preferable range of theTg is no more than 530 degrees C., more preferably is no more than 510degrees C.

Production Method

The optical glasses according to the present invention may be producedby conventional methods of producing optical glasses without limitation,for example, may be produced by the method described below. Each of theraw materials such as oxides, carbonates, nitrates, phosphates, sulfatesand fluoride salts is weighed to a predetermined amount and combineduniformly. The combined raw material is poured into a quartz or aluminacrucible and is preliminarily melted, then poured into a gold, platinum,platinum alloy, or iridium crucible and melted within a melting furnaceat 850 to 1250 degrees C. for 1 to 10 hours. Then the material is mixedand homogenized, followed by cooling to an appropriate temperature andcasting within a mold etc. thereby to produce the glass.

Reheating Test

The glasses free from devitrification within the glass body or thosehaving a transmissivity loss of no more than 5% in the reheating test(a) or (b) may expand freedom in optical design. Furthermore, thechromatic aberration, which conventionally having been reduced by use ofcomplicated processing of lens shape represented by aspheric processingor by way of increasing the number of lenses, may be effectively reducedwithout such complicated processing of lens shape or increasing thenumber of lenses, and also the reheating treatment represented byreheating press processing may be carried out easily, thereby theproduction cost of the optical elements may be saved.

The reheating test (a) is carried out as following: a test piece of aprismatic shape glass sample of 15 mm by 15 mm by 30 mm is set on arefractory body and disposed in an electric furnace, then is reheated.The heating cycle is such that the sample is heated from the ambienttemperature to the temperature 80 degrees C. higher than the grasstransition temperature (Tg) of the sample through 150 minutes, then thesample is maintained at the temperature for 30 minutes; thereafter thesample is allowed to cool to the ambient temperature, and removedoutside the furnace. After polishing the opposing two sides of the testpiece into 10 mm thick, the glass sample is observed visually.

The expression “free from devitrification within the glass body” in thistest means that the processes of heating and softening the cut and/orpolished gobs or glass blocks then press-molding by use of a mold havinga highly precise surface and/or the process of reheat-press processingmay be easily carried out, which is an important property for thepresent invention. In the case of reheat-press processing, the higher isthe temperature set at the reheating test, the lower is the glassviscosity, thus the pressing pressure may be reduced. However, thedurability tends to be deteriorated remarkably for the press-moldedproducts, therefore, the evaluation is preferably carried out under thecondition that the preset temperature is controlled at 50 to 200 degreesC. higher than the glass transition temperature and the duration ofkeeping at the temperature is 5 minutes to 1 hour. More preferably, theevaluation is carried out under the condition that the presettemperature is controlled at 70 to 180 degrees C. higher than the glasstransition temperature and the duration of keeping at the temperature is10 to 40 minutes.

In addition, such a property is necessary for achieving production ofoptical elements with lower cost and proper productivity, such that atleast one of the characteristics of there being substantially noopacification and being devitrified exists in the glass body even aftercertain conditions of reheating test (a) in particular after maintainingat 100 degrees C. higher than the glass transition temperature (Tg) for30 minutes. More preferable is that at least one of the characteristicsof there being substantially no opacification and/or being devitrifiedexists in the glass body even after maintaining at 150 degrees C. higherthan the glass transition temperature (Tg) for 30 minutes.

In addition, it is preferred that value, calculated by dividing thetransmissivity of the test piece after the reheating test (a) by thetransmissivity of the test piece before the reheating test (a) using aradiation (D ray) of wavelength 587.56 nm, is no less than 0.95, morepreferably no less than 0.96 and most preferably no less than 0.97.Furthermore, it is preferred that the difference of the wavelength λ₇₀of the test piece before the reheating test (a) and the wavelength λ₇₀after the reheating test (a) is no more than 20 nm, more preferably nomore than 18 nm and most preferably no more than 16 nm.

The reheating test (b) is carried out in a way that a two side-polishedtest piece of 10 mm thick is heated from room temperature to the yieldpoint at a rising rate of 6.5 degrees C. per second under non-oxidizingatmosphere, then is maintained at the yield point for 300 seconds, andthe temperature is lowered to 220 degrees C. at a rate of 2.4 degrees C.per second, thereafter the transmissivity of the test piece of 10 mmthick is measured in the thickness direction to determine thetransmissivities of before and after the test.

In the present invention, “transmissivity loss” is employed as an indexfor transmissivity degradation under the reheating test (b). The“transmissivity loss” corresponds to the value expressed by percentageof the difference of transmissivities measured at an identicalwavelength within visible irradiation of 360 to 800 nm at which thetransmissivities display the highest difference between before and afterthe reheating test (b). That is to say, transmissivity curves areprepared at visible wavelengths and compared for the samples before andafter the reheating test (b), then the highest difference (%) oftransmissivities at certain wavelength×nm is defined as the“transmissivity loss”. In the present invention, the transmissivity lossis preferably no more than 5%, more preferably no more than 4% and mostpreferably no more than 3%.

In the present invention, “chemical durability”, in particular waterresistance is considered to represent an intimate relation with thetransmissivity degradation in the reheating test. The glasses accordingto the present invention represent weight loss, measured by “In terms ofa Chemical Durability of Optical Glass” JOGIS06-1996 specified byJapanese Optical Glass Industrial Standards, of preferably no more than0.2% by weight, more preferably no more than 0.19% by weight and mostpreferably no more than 0.18% by weight.

The optical glasses of the present invention may be typically utilizedfor lenses, prisms and mirrors. In addition, the method of producingoptical elements according to the present invention typically produces aspherical preform by flowing dropwise a melted glass from an outlet ofoutflow pipe formed of platinum and the like. The preform is subjectedto a precision press-molding process to produce an optical elementhaving an intended shape.

EXAMPLES

The present invention will be explained in more detail with reference toExamples and Comparative Examples, but the present invention will not belimited the Examples.

Each of a total amount of 400 g was weighed according to thecompositions shown in Tables 1 to 6 as raw materials and was mixeduniformly. Each of the raw materials was melted at 950 to 1050 degreesC. for 2 to 3 hours using a quartz or platinum crucible, then thetemperature was lowered to 800 to 900 degrees C. and the material wasmaintained additionally at the temperature for about 1 hour followed bycasting into a mold to produce the respective glasses. The glassproperties of the resulting glasses are shown in Tables 1 to 6. Theglasses of Comparative Examples 1 and 2 having composition shown inTable 7 were also produced using the same processes with those ofExamples.

The optical glasses of Examples were determined for refractive index(n_(d)), Abbe number (ν_(d)) and glass transition temperature (Tg), andthe glasses were subjected to the reheating test.

The refractive index (n_(d)) and Abbe number (ν_(d)) were determined forthe resulting glasses with controlling the slow-coolingtemperature-dropping rate at minus 25 degrees C./hr.

The glass transition temperature (Tg) was determined using athermodilatometer with controlling the temperature-rising rate at 8degrees C./min.

In the reheating test (a), a test piece of 15 mm by 15 mm by 30 mm wasdisposed on a concave refractory and inserted into an electric heaterthen was reheated in a way that the temperature was raised from roomtemperature to a temperature of 80 degrees C. higher than the transitiontemperature (Tg) of each sample, i.e. the temperature at which eachsample sinking into the refractory, through a period of 150 minutes. Thesample was maintained at the temperature for 30 minutes then was cooledto the ambient temperature and was removed from the furnace. Theopposing two sides of each sample were polished to 10 mm thick so as toobserve the inside body, and the polished sample was observed visually.

The transmissivity was measured in accordance with JOGIS02-2003specified by Japanese Optical Glass Industrial Standards. In the presentinvention, the transmissivity was represented rather than color degree.Specifically, an article of which the opposing sides being polished inparallel to 10±0.1 mm thick was determined for the spectral transmissionfactor of D ray. (D ray transmissivity after reheating test (a))/(D raytransmissivity before reheating test (a)) was obtained, and the changeof the maximum transmissivity was evaluated between before and after thereheating test (a).

At the same time, the identical sample was measured as to the differencebetween the λ₇₀ of the test piece before the reheating test (a) and theλ₇₀ of the test piece after the reheating test (a), and the differencewas considered as an index of the transmissivity degradation. The λ₇₀refers to the wavelength at which the transmissivity comes to 70% whenthe transmissivity being measured at various wavelengths in accordancewith JOGIS02-2003. That is to say, the less is the difference betweenthe λ₇₀ of the test piece before the reheating test (a) and the λ₇₀ ofthe test piece after the reheating test (a), the less is thetransmissivity degradation at the reheating test (a).

The condition of the crystal deposition was measured in accordance withJOGIS13-1994 “Method of Determining Foreign Substance within OpticalGlass” specified by Japanese Optical Glass Industrial Standards.Specifically, the test piece after the reheating test was evaluated withrespect to the particle size and the number of foreign substances by useof a microscope capable of detecting and measuring at least 2micrometers or other equipment equivalent therewith. The total crosssection and total number were counted as to glasses of each 100 ml andthey were rated. The 1st grade indicates the total cross section beingless than 0.03 mm², the 2nd grade indicates the total cross sectionbeing from no less than 0.03 mm² to less than 0.1 mm², the 3rd gradeindicates the total cross section being from no less than 0.1 mm² toless than 0.25 mm², the 4th grade indicates the total cross sectionbeing from no less than 0.25 mm² to less than 0.5 mm² and the 5th gradeindicates the total cross section being no less than 0.5 mm² in a glassof 100 ml respectively. The A grade indicates the total number beingless than 10, the B grade indicates the total number being from no lessthan 10 to less than 100, the C grade indicates the total number beingfrom no less than 100 to less than 500, the D grade indicates the totalnumber being from no less than 500 to less than 1000 and the E gradeindicates the total number being no less than 1000.

The reheating test (b) was carried out in a way that a two side-polishedtest piece of 10 mm thick was heated from room temperature to the yieldpoint at a rising rate of 6.5 degrees C. per second under non-oxidizingatmosphere, then was maintained at the yield point for 300 seconds, andthe temperature was lowered to 220 degrees C. at a rate of 2.4 degreesC. per second, thereafter the transmissivity of the test piece wasmeasured in the thickness direction to determine the transmissivities ofbefore and after the test. The transmissivity degradation means that thetest piece after the test shows a lower transmissivity compared to thatbefore the test through undergoing the reheating test (b).

In the present invention, “transmissivity loss” is employed as an indexfor degradation of transmissivity under the reheating test (b). The“transmissivity loss” corresponds to the value expressed by percentageof the difference of transmissivities measured at an identicalwavelength within visible irradiation of 360 to 800 nm at which thetransmissivities display the highest difference between before and afterthe reheating test (b). That is to say, transmissivity curves areprepared at visible wavelengths and compared for the samples before andafter the reheating test (b), then the highest difference (%) oftransmissivities at certain wavelength×nm is defined as the“transmissivity loss”.

The chemical durability or water resistance was determined in accordancewith “Method of Determining Chemical Durability of Optical Glass”JOGIS06-1996 specified by Japanese Optical Glass Industrial Standards.The glass weight loss means the value, expressed as % by weight, of theglass weight reduced through the chemical durability test.

One gravity gram of glass sample, fractured into 425 to 600 micrometergrit, was weighed and put into a platinum cage. The platinum cage wasinserted into a quartz-glass round-bottom flask containing-pure water ofpH 6.5 to 7.5, then was treated in a boiling-water bath for 60 minutes.The weight loss % of the treated glass samples was calculated and ratedsuch that the weight loss (wt %) of less than 0.05 being class 1, theweight loss of from 0.05 to less than 0.10 being class 2, the weightloss of from 0.10 to less than 0.25 being class 3, the weight loss offrom 0.25 to less than 0.60 being class 4, the weight loss of from 0.60to less than 1.10 being class 5 and the weight loss of no less than 1.10being class 6; the lower is the class number, more superior is the waterresistance of the glass.

-   Table 1-   Table 2-   Table 3-   Table 4-   Table 5-   Table 6-   Table 7

The inventive glasses of Examples 1 to 57 exhibited lower glasstransition temperatures compared to the glasses of Comparative Examples1 and 2, displayed almost no crystal deposition, and were colorless andtransparent, and also showed almost no change in the maximumtransmissivity even after the reheating test. Furthermore, the glassesof Examples 55 to 57 represented the values of Al₂O₃+ZrO₂+SiO₂+Bi₂O₃ and(Al₂O₃+ZrO₂+SiO₂)/RO higher than a certain level, and exhibited betterresults in terms of reheating tests (a) and (b) compared to the glass ofComparative Example 3.

While preferred embodiments of the present invention have been describedand illustrated above, it is to be understood that they are exemplary ofthe invention and are not to be considered to be limiting. Additions,omissions, substitutions, and other modifications can be made theretowithout departing from the spirit or scope of the present invention.Accordingly, the invention is not to be considered to be limited by theforegoing description and is only limited by the scope of the appendedclaims. TABLE 1 Examples 1 2 3 4 5 6 7 8 9 SiO₂ 4.489 4.998 4.704 4.3734.023 4.197 3.785 3.846 4.398 B₂O₃ 11.691 10.408 12.483 11.606 16.30016.999 15.334 15.578 11.671 SiO₂ + B₂O₃ 16.180 15.406 17.187 15.97920.323 21.196 19.119 19.424 16.069 SiO₂/B₂O₃ 0.384 0.480 0.377 0.3770.247 0.247 0.247 0.247 0.377 Al₂O₃ Y₂O₃ La₂O₃ Gd₂O₃ Yb₂O₃ TiO₂ 2.5872.343 2.658 0.989 ZrO₂ Nb₂O₅ 1.688 3.464 3.289 Ta₂O₅ WO₃ 0.589 1.5431.442 ZnO MgO 0.788 1.609 CaO SrO 3.316 BaO 19.995 15.986 15.853 23.27630.800 26.775 26.084 21.592 25.315 RO 19.995 16.774 17.462 23.276 30.80026.775 26.084 24.908 25.315 Li₂O 0.949 1.363 0.994 0.924 2.000 3.1311.883 1.913 0.930 Na₂O 2.952 3.231 3.299 2.684 3.085 K₂O 2.691 3.6831.880 1.748 1.758 Rn₂O 6.592 8.277 6.173 5.356 2.000 3.131 1.883 1.9135.773 RO + Rn₂O 26.587 25.051 23.635 28.632 32.800 29.906 27.967 26.82131.088 Sb₂O₃ 0.097 0.090 0.078 0.081 0.073 0.075 0.091 P₂O₅ Bi₂O₃ 52.36853.737 54.880 51.021 46.800 48.817 52.841 53.681 51.310 GeO₂ F TOTAL100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Al₂O₃ +ZrO₂ + SiO₂ + Bi₂O₃ 56.857 58.735 59.584 55.394 50.823 53.014 56.62657.527 55.708 (Al₂O₃ + ZrO₂ + SiO₂)/RO 0.225 0.298 0.269 0.188 0.1310.157 0.145 0.154 0.174 n_(d) 1.848 1.848 1.847 1.848 1.825 1.816 1.8531.854 1.828 ν_(d) 24.4 23.8 24.4 25.2 28.2 28.4 26.3 26.0 25.7 Tg (° C.)367 385 415 416 431 423 417 425 386 Reheating crystal deposit 1A 1A 1A1A 1A 1A 1A 1A 1A test (a) condition D ray 1 0.993 1 1.003 1 1 1 1 1transmissivity after test/D ray transmissivity before test (%) λ₇₀ aftertest − before 15.5 16 test λ₇₀ (nm) Reheating transmissivity test (b)loss(%) glass weight loss(wt %)

TABLE 2 Examples 10 11 12 13 14 15 16 17 18 SiO₂ 3.805 4.316 4.246 4.5014.650 4.568 4.378 4.593 B₂O₃ 15.429 11.049 16.651 11.265 11.944 12.34012.572 11.618 11.735 SiO₂ + B₂O₃ 19.234 15.365 16.651 15.511 16.44516.990 17.140 15.996 16.328 SiO₂/B₂O₃ 0.247 0.391 0.377 0.377 0.3770.363 0.377 0.391 Al₂O₃ Y₂O₃ La₂O₃ Gd₂O₃ Yb₂O₃ TiO₂ 2.543 3.614 0.4944.153 ZrO₂ Nb₂O₅ 3.366 3.292 Ta₂O₅ WO₃ 1.432 1.393 1.476 1.525 1.4981.506 ZnO MgO 4.242 CaO SrO BaO 19.419 25.778 25.140 24.436 21.026 7.60218.367 23.301 16.474 RO 19.419 25.778 25.140 24.436 21.026 11.844 18.36723.301 16.474 Li₂O 1.892 0.912 0.923 0.951 0.983 0.965 0.925 0.971 Na₂O3.027 3.063 3.157 3.261 3.204 3.071 3.221 K₂O 2.982 1.725 1.746 9.0501.799 1.859 1.826 1.750 3.672 Rn₂O 4.874 5.664 5.732 9.050 5.907 6.1035.995 5.745 7.864 RO + Rn₂O 24.293 31.441 30.872 33.486 26.933 17.94724.362 29.046 24.338 Sb₂O₃ 0.090 0.087 0.093 0.096 0.094 0.090 0.095P₂O₅ Bi₂O₃ 53.107 53.196 50.955 49.527 52.510 63.442 53.292 51.07853.580 GeO₂ F TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00100.00 100.00 Al₂O₃ + ZrO₂ + SiO₂ + Bi₂O₃ 56.912 57.512 50.955 53.77357.011 68.092 57.860 55.456 58.173 (Al₂O₃ + ZrO₂ + SiO₂)/RO 0.196 0.1670.000 0.174 0.214 0.393 0.249 0.188 0.279 n_(d) 1.837 1.839 1.814 1.8201.841 1.834 1.858 1.842 1.844 ν_(d) 26.0 25.5 26.4 25.8 24.9 24.4 24.225.3 23.9 Tg (° C.) 423 385 393 424 408 408 421 417 397 Reheatingcrystal deposit 1A 1A 1A 1A 1A 1A 1A 1A 1A test (a) condition D ray 1 11 1 1 1 1 1 1 transmissivity after test/D ray transmissivity before test(%) λ₇₀ after test − before test λ₇₀ (nm) Reheating transmissivity test(b) loss(%) glass weight loss(wt %)

TABLE 3 Examples 19 20 21 22 23 24 25 26 27 SiO₂ 4.445 4.187 4.254 4.6294.627 4.602 4.202 3.857 3.925 B₂O₃ 11.795 11.111 8.775 12.282 12.05011.759 11.734 15.625 15.900 SiO₂ + B₂O₃ 16.240 15.298 13.029 16.91116.677 16.361 15.936 19.482 19.825 SiO₂/B₂O₃ 0.377 0.377 0.485 0.3770.384 0.391 0.358 0.247 0.247 Al₂O₃ Y₂O₃ La₂O₃ Gd₂O₃ Yb₂O₃ TiO₂ 2.0090.946 3.924 4.184 4.265 4.152 ZrO₂ Nb₂O₅ 1.671 3.149 Ta₂O₅ WO₃ 1.2071.506 ZnO MgO 1.316 CaO SrO 8.105 BaO 22.691 20.196 20.491 17.068 17.06216.507 16.472 21.657 22.038 RO 22.691 20.196 28.596 17.068 17.062 16.50716.472 21.657 23.354 Li₂O 0.939 0.885 2.158 0.978 0.978 0.973 0.9711.918 1.952 Na₂O 2.728 2.570 3.246 3.245 3.228 3.623 K₂O 1.777 1.6743.700 3.699 3.679 3.671 3.024 Rn₂O 5.444 5.129 2.158 7.924 7.922 7.8808.265 4.942 1.952 RO + Rn₂O 28.135 25.325 30.754 24.992 24.984 24.38724.737 26.599 25.306 Sb₂O₃ 0.092 0.086 0.140 0.172 0.172 0.095 0.0950.075 0.076 P₂O₅ Bi₂O₃ 51.853 55.196 56.077 54.001 53.983 53.686 53.57453.844 54.793 GeO₂ F TOTAL 100.00 100.00 100.00 100.00 100.00 100.00100.00 100.00 100.00 Al₂O₃ + ZrO₂ + SiO₂ + Bi₂O₃ 56.298 59.383 60.33058.630 58.610 58.288 57.776 57.701 58.718 (Al₂O₃ + ZrO₂ + SiO₂)/RO 0.1960.207 0.149 0.271 0.271 0.279 0.255 0.178 0.168 n_(d) 1.856 1.876 1.8751.836 1.843 1.846 1.839 1.819 1.854 ν_(d) 24.4 23.5 24.6 24.4 23.9 23.824.1 26.8 25.9 Tg (° C.) 404 404 405 399 397 389 397 416 409 Reheatingcrystal deposit 1A 1A 1A 1A 1A 1A 1A 1A 1A test (a) condition D ray 1 11 1 1 1 1 1 0.994 transmissivity after test/D ray transmissivity beforetest (%) λ₇₀ after test − before −7.5 test λ₇₀ (nm) Reheatingtransmissivity test (b) loss(%) glass weight loss(wt %)

TABLE 4 Examples 28 29 30 31 32 33 34 35 36 SiO₂ 3.984 4.960 4.565 4.6004.675 4.417 4.637 4.626 4.698 B₂O₃ 16.140 13.185 12.114 12.207 10.56511.722 11.848 11.818 10.615 SiO₂ + B₂O₃ 20.124 18.145 16.679 16.80715.240 16.139 16.486 16.444 15.313 SiO₂/B₂O₃ 0.247 0.376 0.377 0.3770.442 0.377 0.391 0.391 0.443 Al₂O₃ Y₂O₃ La₂O₃ Gd₂O₃ Yb₂O₃ TiO₂ 4.1274.159 1.585 4.717 4.444 2.389 ZrO₂ Nb₂O₅ 1.758 3.322 3.533 Ta₂O₅ WO₃1.627 1.497 1.509 0.767 ZnO MgO 4.524 2.132 1.071 CaO SrO 6.871 BaO12.202 11.335 18.355 16.500 15.249 23.508 16.602 17.056 13.247 RO 19.07315.859 18.355 16.500 17.381 23.508 16.602 17.056 14.318 Li₂O 1.982 1.0480.965 0.972 0.988 0.934 0.980 0.978 1.787 Na₂O 3.478 3.202 3.226 3.2792.710 3.253 3.244 3.295 K₂O 3.123 1.982 1.825 3.065 4.361 1.766 3.7073.698 4.382 Rn₂O 5.105 6.508 5.992 7.263 8.628 5.410 7.940 7.920 9.464RO + Rn₂O 24.178 22.367 24.347 23.763 26.009 28.918 24.543 24.976 23.782Sb₂O₃ 0.077 0.094 0.095 0.096 0.091 0.153 0.172 0.174 P₂O₅ Bi₂O₃ 55.62157.861 53.256 53.667 54.545 51.529 54.098 53.964 54.809 GeO₂ F TOTAL100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Al₂O₃ +ZrO₂ + SiO₂ + Bi₂O₃ 59.605 62.821 57.821 58.267 59.220 55.946 58.73558.590 59.507 (Al₂O₃ + ZrO₂ + SiO₂)/RO 0.209 0.313 0.249 0.279 0.2690.188 0.279 0.271 0.328 n_(d) 1.816 1.824 1.867 1.848 1.828 1.848 1.8561.856 1.839 ν_(d) 27.0 24.8 23.8 24 24.3 24.9 23.2 23.2 23.9 Tg (° C.)419 393 421 409 385 395 397 395 381 Reheating crystal deposit 1A 1A 1A1A 1A 1A 1A 1A 1A test (a) condition D ray 1 1 0.981 1 1 1 1 1 1.007transmissivity after test/D ray transmissivity before test (%) λ₇₀ aftertest − before 6 15 test λ₇₀ (nm) Reheating transmissivity test (b)loss(%) glass weight loss(wt %)

TABLE 5 Examples 37 38 39 40 41 42 43 44 45 SiO₂ 4.477 4.463 4.504 4.5034.603 4.615 4.518 4.445 4.433 B₂O₃ 11.905 8.328 10.179 10.176 11.76211.790 11.567 8.294 10.016 SiO₂ + B₂O₃ 16.382 12.791 14.683 14.67916.365 16.405 16.085 12.739 14.449 SiO₂/B₂O₃ 0.376 0.536 0.442 0.4430.391 0.391 0.391 0.536 0.443 Al₂O₃ Y₂O₃ La₂O₃ Gd₂O₃ Yb₂O₃ TiO₂ 2.5223.309 3.308 3.902 4.172 5.107 2.512 3.256 ZrO₂ Nb₂O₅ Ta₂O₅ WO₃ 1.4681.464 1.477 1.477 1.458 1.454 ZnO MgO 4.085 CaO SrO BaO 25.727 20.85019.576 18.778 17.973 17.516 17.187 20.766 18.466 RO 29.811 20.850 19.57618.778 17.973 17.516 17.187 20.766 18.466 Li₂O 0.943 0.952 0.952 0.9730.975 0.955 0.939 0.937 Na₂O 3.913 3.159 3.158 3.229 3.237 3.169 3.1183.109 K₂O 5.352 4.201 4.600 3.680 3.689 1.806 6.515 4.134 Rn₂O 10.2088.312 8.710 7.882 7.901 5.930 10.572 8.180 RO + Rn₂O 29.811 31.05827.888 27.488 25.855 25.417 23.117 31.338 26.646 Sb₂O₃ 0.092 0.092 0.0930.117 0.171 0.171 0.092 0.146 P₂O₅ Bi₂O₃ 52.242 52.072 52.550 52.93153.706 53.835 55.691 51.861 54.049 GeO₂ F TOTAL 100.00 100.00 100.00100.00 100.00 100.00 100.00 100.00 100.00 Al₂O₃ + ZrO₂ + SiO₂ + Bi₂O₃56.719 56.535 57.054 57.434 58.309 58.450 60.209 56.306 58.482 (Al₂O₃ +ZrO₂ + SiO₂)/RO 0.150 0.214 0.230 0.240 0.256 0.263 0.263 0.214 0.240n_(d) 1.867 1.829 1.829 1.835 1.840 1.845 1.880 1.842 1.853 ν_(d) 25.424.3 24.2 24.2 24.2 23.8 22.8 23.9 23.2 Tg (° C.) 416 379 394 405 405389 426 374 372 Reheating crystal deposit 1A 1A 1A 1A 1A 1A 1A 1A 1Atest (a) condition D ray 1 1 1 1 1 1 1 0.977 1.007 transmissivity aftertest/D ray transmissivity before test (%) λ₇₀ after test − before −1.5 8test λ₇₀ (nm) Reheating transmissivity test (b) loss(%) glass weightloss(wt %)

TABLE 6 Examples 46 47 48 49 50 51 52 53 54 SiO₂ 4.467 2.954 4.025 3.9002.460 4.315 4.353 4.476 2.849 B₂O₃ 10.093 8.900 7.772 7.530 7.127 8.6668.743 8.989 9.904 SiO₂ + B₂O₃ 14.560 11.854 11.797 11.430 9.587 12.98113.096 13.465 12.753 SiO₂/B₂O₃ 0.443 0.332 0.518 0.518 0.345 0.498 0.4980.498 0.288 Al₂O₃ Y₂O₃ La₂O₃ 6.408 3.119 Gd₂O₃ 3.502 Yb₂O₃ TiO₂ 3.2820.794 ZrO₂ 0.590 Nb₂O₅ Ta₂O₅ WO₃ 1.465 ZnO 4.001 3.634 3.320 3.117 3.9314.041 1.930 MgO CaO SrO 1.502 1.544 1.474 BaO 18.991 7.539 6.635 2.9365.090 RO 18.991 11.540 3.634 6.635 3.320 6.053 5.433 5.585 8.494 Li₂O0.944 1.469 1.334 1.293 1.224 1.431 1.443 1.484 1.417 Na₂O 3.133 K₂O4.166 Rn₂O 8.243 1.469 1.334 1.293 1.224 1.431 1.443 1.484 1.417 RO +Rn₂O 27.234 13.009 4.968 7.928 4.544 7.484 6.876 7.069 9.911 Sb₂O₃ 0.166P₂O₅ Bi₂O₃ 53.293 68.729 83.234 80.643 85.866 75.821 76.526 78.67377.336 GeO₂ F TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00100.00 100.00 Al₂O₃ + ZrO₂ + SiO₂ + Bi₂O₃ 57.76 71.683 87.259 84.54388.326 80.726 80.879 83.149 80.185 (Al₂O₃ + ZrO₂ + SiO₂)/RO 0.235 0.2561.108 0.588 0.741 0.810 0.801 0.801 0.335 n_(d) 1.848 1.969 2.062 2.0321.848 2.031 2.034 2.043 2.007 ν_(d) 23.5 21.6 18.3 18.9 23.5 19.4 19.218.7 19.6 Tg (° C.) 395 415 388 392 395 388 385 383 380 Reheatingcrystal deposit 1A 1A 1A 1A 1A 1A 1A 1A 1A test (a) condition D ray 1 11 1 1 1 1 1 1 transmissivity after test/D ray transmissivity before test(%) λ₇₀ after test − before test λ₇₀ (nm) Reheating transmissivity test(b) loss(%) glass weight loss(wt %)

TABLE 7 Examples Comparative Examples 55 56 57 58 59 60 1 2 3 SiO₂ 5.8565.861 5.861 4.82 4.860 5.032 4.883 4.37 B₂O₃ 13.226 12.236 13.236 12.7812.922 15.335 26.000 12.981 11.61 SiO₂ + B₂O₃ 19.082 18.097 19.097 17.6017.782 20.367 26.000 17.864 15.979 SiO₂/B₂O₃ 0.443 0.479 0.443 0.3770.376 0.328 0.000 0.376 0.376 Al₂O₃ 5.538 5.542 6.542 10.000 Y₂O₃ La₂O₃10.000 Gd₂O₃ Yb₂O₃ TiO₂ 1.09 0.99 ZrO₂ 0.999 2.000 Nb₂O₅ 3.62 3.672 3.29Ta₂O₅ WO₃ 1.594 1.601 ZnO 6.098 4.000 MgO 4.434 4.454 CaO 11.27 11.2794.203 SrO 10.97 BaO 8.994 9.40 17.434 6.035 9.041 23.28 RO 11.27 11.2798.994 20.374 21.868 16.336 4.000 13.495 23.276 Li₂O 1.02 2.054 1.0631.032 0.92 Na₂O 3.529 3.424 2.68 K₂O 1.952 1.748 Rn₂O 0 0 0 1.02 2.054.59 6.408 5.356 RO + Rn₂O 11.27 11.279 0 21.394 23.918 20.926 4.00019.903 32.632 Sb₂O₃ 0.16 0.082 0.082 0.1 0.09 P₂O₅ Bi₂O₃ 62.95 63.00065.285 56.2 56.702 58.705 50.000 56.960 51.02 GeO₂ F TOTAL 100.00 100.00100.00 100.00 100.00 100.00 100.00 100.00 100.00 Al₂O₃ + ZrO₂ + SiO₂ +Bi₂O₃ 75.344 76.403 77.688 61.020 61.562 63.737 60.000 61.843 55.390(Al₂O₃ + ZrO₂ + SiO₂)/RO 1.100 1.188 1.379 0.237 0.222 0.308 2.500 0.3620.188 n_(d) 1.855 1.86 1.856 1.913 1.866 1.836 1.748 1.847 1.857 ν_(d)25.6 25.4 24.5 23.4 25 25.6 32.8 23.7 24.5 Tg (° C.) 492 503 458 453 519439 415 Reheating crystal deposit 1A 1A 1A 1A 1A 1A no data non- 1A test(a) condition due to detectable higher due Tg to opacifi D ray 1 1 10.995 1.008 1.002 no data 0 1 transmissivity due to after test/D rayhigher transmissivity Tg before test (%) λ₇₀ after test − before −13.516 15.5 test λ₇₀ (nm) Reheating transmissivity 1 0.7 0.9 22.27 test (b)loss(%) glass weight 0.03 0.02 0.02 0.36 loss(wt %)

1. An optical glass comprising: a refractive index (n_(d)) of no lessthan 1.75 and an Abbe number (ν_(d)) of no less than 10 as opticalconstants, wherein a Bi₂O₃ content is no less than 10% by weight to nomore than 90% by weight, and the optical glass has at least one of theproperties of being substantially free from opacification and/or beingsubstantially devitrified within the glass body under the conditions ofthe following reheating test (a), whereby, a test piece of 15 mm by 15mm by 30 mm is reheated, such that the test piece is heated from roomtemperature to a temperature of 80 degrees C. higher than its transitiontemperature (Tg) for a period of 150 minutes, maintained for 30 minutesat the temperature of 80 degrees C. higher than the glass transitiontemperature (Tg) of the optical glass, allowed to cool to an ambienttemperature, and finally observed visually after polishing the opposingtwo sides of the test piece to a thickness of 10 mm.
 2. The opticalglass according to claim 1, wherein the transmissivity loss is no morethan 5% at respective wavelengths in the visible region under theconditions of the following reheating test (b), whereby a twosided-polished test piece having a thickness of 10 mm is heated fromroom temperature to a yield point by increasing the temperature at arate of 6.5 degrees C. per second under a non-oxidizing atmosphere,being maintained at the yield point for 300 seconds, lowering thetemperature to 220 degrees C. by decreasing the temperature at a rate of2.4 degrees C. per second, and thereafter measuring the transmissivityof the test piece to determine the transmissivity before and after thetest.
 3. The optical glass according to claim 1, wherein a value,calculated by dividing the transmissivity of the test piece after thereheating test (a) by the transmissivity of the test piece before thereheating test, using a radiation (D ray) at a wavelength of 587.56 nm,is no less than 0.95.
 4. The optical glass according to claim 1, whereinthe difference in a wavelength λ₇₀ of the test piece before thereheating test (a) and a wavelength λ₇₀ after the reheating test is nomore than 20 nm, in which the “λ₇₀” refers to the wavelength at whichthe transmissivity being 70%.
 5. The optical glass according to claim 1,wherein the crystal deposit condition of the test piece after thereheating test (a) displays an internal quality of a first or secondgrade and A or B grade by an evaluation which is in accordance with ameasuring method for inclusion JOGIS13-1994.
 6. The optical glassaccording to claim 1, wherein the transition temperature (Tg) of theglass is no more than 550 degrees C.
 7. The optical glass according toclaim 1, wherein the content of SiO₂ is lower than the content of B₂O₃,the total content of SiO₂+B₂O₃ is from no less than 1% by weight to nomore than 60% by weight; and the total content of TiO₂+Nb₂O₅+WO₃+RO+Rn₂Ois no more than 60% by weight, in which R represents one or moreelements selected from the group consisting of Zn, Ba, Sr, Ca and Mg; Rnrepresents one or more elements selected from the group consisting ofLi, Na, K and Cs.
 8. The optical glass according to claim 1, wherein thecontent of SiO₂ is lower than the content of B₂O₃, the total content ofSiO₂+B₂O₃ is from no less than 1% by weight to no more than 60% byweight; and the total content of TiO₂+Nb₂O₅+WO₃+RO+Rn₂O is from no lessthan 0.1% by weight to no more than 55% by weight, in which R representsone or more selected elements selected from the group consisting of Zn,Ba, Sr, Ca and Mg; and Rn represents one or more elements selected fromthe group consisting of Li, Na, K and Cs.
 9. The optical glass accordingto claim 1, wherein the content of SiO₂ is lower than the content ofB₂O₃, the total content of SiO₂+B₂O₃ is from no less than 1% by weightto no more than 60% by weight; and the total content of RO+Rn₂O is fromno less than 0.1% by weight to no more than 60% by weight, in which Rrepresents one or more elements selected from the group consisting ofZn, Ba, Sr, Ca and Mg; and Rn represents one or more elements selectedfrom the group consisting of Li, Na, K and Cs.
 10. The optical glassaccording to claim 1, wherein the content of SiO₂ is lower than thecontent of B₂O₃, the total content of SiO₂+B₂O₃ is from no less than 1%by weight to no more than 60% by weight; and the total content ofLn₂O₃+RO+Rn₂O is from no less than 0.5% by weight to no more than 50% byweight, in which R represents one or more elements selected from thegroup consisting of Zn; Ba, Sr, Ca and Mg; Rn represents one or moreelements selected from the group consisting of Li, Na, K and Cs; and Lnrepresents one or more elements selected from the group consisting ofLa, Gd, Y, Ce, Eu, Dy, Yb and Lu.
 11. The optical glass according toclaim 1, wherein the content of MgO is less than 4% by weight, and thecontent of TiO₂+Nb₂O₅+WO₃+Ln₂O₃ is no more than 10% by weight, in whichLn represents one or more elements selected from the group consisting ofLa, Gd, Y, Ce, Eu, Dy, Yb and Lu.
 12. The optical glass according toclaim 1, wherein the content of Rn₂O is from 0% by weight to 1.5% byweight, in which Rn represents one or more elements selected from thegroup consisting of Li, Na, K and Cs.
 13. The optical glass according toclaim 1, wherein the total content of Bi₂O₃+SiO₂+Al₂O₃+ZrO₂ is no lessthan 75% by weight.
 14. The optical glass according to claim 1, whereinthe weight loss of the glass is no more than 0.2% by weight in terms ofa chemical durability test based on a powder method in accordance withJOGIS06-1996.
 15. The optical glass according to claim 1, wherein avalue of (SiO₂+Al₂O₃+ZrO₂)/RO is no less than 0.5.
 16. An opticalelement formed by precision press-molding of the optical glass accordingto claim
 1. 17. A preform utilized for precision press-moldingcomprising of the optical glass according to claim
 1. 18. An opticalelement formed by precision press-molding of the preform utilized forprecision press-molding according to claim 17.